1. Field of the Invention
The present invention relates to a process for forming stacks of printed products, especially books, magazines, newspapers, brochures, etc., from a plurality of printed sheets which have been supplied by a conveying section and assembled thereon into preliminary products, wherein the preliminary products are then processed into stacks. The invention also relates and a system for implementing the process.
2. Description of the Related Art
Printed products, especially books, magazines, newspapers, brochures, and similar products are produced today primarily on production lines on an industrial basis. These types of production lines are formed by individual machines, which are arranged in a series, one behind the other. The individual machines are connected to each other. Each of these individual machines can have a maximum production speed, which depends on the product parameters and on the production conditions. The maximum possible production speed of the overall production line is therefore limited by the machine with the slowest maximum speed. The situation becomes especially serious when product parameters change constantly during production, i.e., parameters which influence the maximum production speed of the machine and limit the maximum production speed of the production line. This is the case, for example, in a stacking device which, as a function of the quantities ordered by different recipients, is called upon to form stacks of different sizes. For a stacking device there are two upper output limits, which cannot be exceeded. The first limit pertains to the maximum possible cadence at which the printed products can be accepted by the stacking device. The second limit pertains to the maximum possible cadence or the minimum possible cycle time at which a stack can be discharged from the stacking device. Because the maximum possible feed cadence is many times higher than the maximum possible discharge cadence, it is impossible to form stacks which are smaller than the quotient, rounded to the next higher whole number, of the maximum possible feed cadence divided by the maximum possible discharge cadence.
According to a first prior art reference, the problem is solved by using a distributing device to distribute the printed products over several parallel stacking devices and by then bringing the stacks back again to one line. With a sufficient number of stacking devices, it is possible to process the entire production output of the rest of the line at any time. The disadvantages, however, are the large number of machines required, the additional amount of space which the machines occupy, and the difficulty with which access can be gained to the individual stacking devices, which are set up parallel to each other.
According to a second prior art reference, the production output of the entire production line is reduced far enough during the formation of small stacks in the stacking device that there is sufficient time for the stacks to be discharged. The disadvantage of this solution, however, is that, as a result of the constant change in the production output, the quality of the printed products and the reliability of the production line can be negatively affected.
According to yet another prior art reference, a product buffer is inserted upstream of the stacking device. This buffer can be filled with printed products during the formation of small stacks and emptied again during the formation of large stacks. This solution also suffers from the disadvantage that a large number of additional machines and control functions are required. Furthermore, the buffer can become overfilled, when a large number of small stacks are to be formed in succession.